Method of polishing methyl methacrylate sheets



United States Patent @ffice 3,107,464 Patented Oct. 22, 1963 3,107,464METHOD OF POLISHING METHYL METHACRYLATE SHEETS Richard L. Cook, Phoenix,Ariz., assignor to Goodyear Aircraft Corporation, Akron, Ohio, acorporation of Delaware No Drawing. Filed Dec. 4, 1961, Ser. No. 156,998

6 Claims. (Cl. 51-323) This invention relates to improved methods forpolishing methyl methacrylate sheets, and, more particularly, isconcerned with the elimination of haze, cracks, and surface crazing, inplastic sheets of the indicated type.

Heretofore, it has been the usual practice in removing haze, and thelike, from methyl methacrylate sheets to grind and polish the surface ofthe sheet for a sufiicient period of time, and to the required depth, toremove the haze, but this often becomes such a laborious andtimeconsuming operation that the operation is so very expensive as to beimpractical.

It is the general object of the invention to avoid and overcome theforegoing and other difficulties of and objections to prior artpractices by the provision of a relatively rapid, inexpensive, andimproved method for elim inating haze, and other surface blemishes, uponmethyl methacrylate sheets.

Another object of the invention is the provision of an improved processfor resurfacing methyl methacrylate sheets, and particularly stretchedsheets of cross linked methyl methacrylate, such as sold under thetrademark Plexiglas 55.

The foregoing objects of the invention, and other objects which willbecome apparent as the description proceeds, are achieved by that methodof resurfacing a stretched methyl methacrylate sheet which includes thesteps of subjecting the surface of the sheet to the action of anon-solvent wetting agent mixed with unpolymerized methyl methacrylatemonomer for a selected period of time, the monomer acting to soften andfill the surface to the desired depth, and the wetting agent acting toslow the evaporation rate of the monomer and to slow its solvent action,gradually terminating the action of the monomer by the application ofadditional wetting agent and fine abrasive powder to the surface, andgrinding the softened surface to the polish desired.

The principles of the invention are particularly applicable to theresurfacing of methyl methacrylate sheets wherein the molecules of thesheets are cross linked, as in Plexiglas 55, and to sheets of this typewhich have been stretched to flat or contoured form. In one particularapplication of the invention stretched airplane canopies of transparentPlexiglas 55 have been resurfaced to an optically satisfactory conditionin a period of time equal to about ten percent of the time that wouldhave been required to resurface the sheets with the conventionalgrinding and polishing operations of the prior art.

In the practice of the invention the methyl methacrylate sheet to beresurfaced is first treated with a solvent A, this constituting amixture of a non-solvent wetting agent, such as kerosene, and an acrylicmonomer, which is a thin, clear, volatile liquid constituting the basicmaterial in unpolymerized form from which the methyl methac rylate sheetis made. In one typical example solvent A constituted:

Percent MMA monomer 50 Kerosene 50 the methyl methacrylate sheet to thedepth of the haze or surface cracks, for example, up to ninety percentof the depth of such haze. Solvent A can be applied to only one surfaceof the sheet by laying the sheet horizontally and applying solvent Athereto in a layer, or by coating the surface to be refinished of acontoured sheet of methyl methacrylate, or by dipping the sheet to beresurfaced into a container containing solvent A.

Usually solvent A is applied to the surface of a severely hazed methylmethacrylate sheet to be refinished for a period of about sixty minutes.However, for less severe conditions of surface hazing, the time that thesheet to be resurfaced is subjected to solvent A can be reduced, or ifthe surface haze is particularly deep, the time of application ofsolvent A can be increased.

The monomer in solvent A acts to soften or swell the methyl methacrylatesheet with the kerosene acting as a wetting agent and acting to retardthe solvent action of the monomer.

After the desired softening of the methyl methacrylate sheet has beenachieved, and without regard to the residual solvent A on the sheet,additional kerosene and fine abrasive powder, such as grit classified A,is added to the sheet and polishing is begun.

Thus, there is a gradual to rapid reduction in the solvent action of themonomer upon the methyl methacrylate sheet concurrently with thepolishing operation, the kerosene acting still as a Wetting agent.

Upon completion of the polishing operation the sheet is washed withnaphtha to remove all oily residue.

Several theories have been advanced concerning the operation and theexplanation of the operation of the process of the invention. In accordwith the most favored theory of operation, solvent A is believed to actas a softening and swelling agent, but with the unpolymerized monomersof the acrylic material present in solvent A acting to fill down intoall cracks and voids in the surface to be refinished and beingpolymerized therein whereby the optical transparency of the surface ismaterially improved. In accord with this theory, the swollen andsoftened surface being refinished, after being filled as described, ismore readily ground and polished than the surface of the methylmethacrylate sheet which has not been swollen and softened, and with afaster and smoother grinding action being achieved because the surfaceto be refinished is still wet with the wetting agent in solvent A whichhas been added to during polishing.

In any event, regardless of the exact technical explanation of whathappens in every step of the process of the invention, it has been foundthat the process of the invention is much more rapid and is much lessexpensive than prior known grinding and polishing operations, and withthe process of the invention resulting in optically transparentresurfaced sheets of methyl methacrylate of commercially satisfactorycharacter. Exposure tests to ultraviolet light and sunlight havedemonstrated that the process of the invention produces refinishedsurfaces on methyl methacrylate sheets which are just as satisfactoryweatherwise and exposure-wise as are refinished surfaces made with priorart grinding and polishing operations.

While a certain representative embodiment and details have been shownfor the purpose of illustrating the invention, it will be apparent tothose skilled in this art that various changes and modifications may bemade therein without departing from the spirit or scope of theinvention.

What is claimed is: Y

1. That method of removing haze and the like from a surface of atransparent cross-linked methyl methacrylate sheet which includes thesteps of applying to the surface of the sheet a solution containingabout 50% of a nonsolvent wetting agent and containing about 50% acrylicmonomers, leaving the solution in contact with the sur face for a periodof time sufficient to swell the sheet a distance substantially equal tothe thickness of the haze, progressively reducing and then terminatingthe action of the solution by the application of additional quantitiesof the wetting agent to the surface, together with abrasive powder,abrasive grinding the surface to the polish desired while continuing toadd the wetting agent and abrasive powder to the surface, and washingthe polished sheet to remove the wetting agent and abrasive powdertherefrom.

2. That method of removing haze and the like from a surface of atransparent cross-linked methyl methacrylate sheet which includes thesteps of applying to the surface of the sheet a solution containingabout 50% of kerosene and containing about 50% acrylic monomers, leavingthe solution in contact with the surface for a period of time sufiicientto swell the sheet a distance substantially equal to the thickness ofthe haze, progressively reducing and then terminating the action of thesolution by the application of additional quantities of kerosene to thesurface together with abrasive powder, abrasive grinding the surface tothe polish desired while continuing to add the kerosene and abrasivepowder to the surface, and washing the polished sheet with naphtha toremove the kerosene and abrasive powder therefrom.

3. That method of removing haze and the like from a surface of atransparent cross-linked methyl methacrylate sheet which includes thesteps of applying to the surface of the sheet a solution comprising anon-solvent wetting agent and acrylic monomers, leaving the solution incontact with the surface for a period of time sufficient to swell thesheet a distance substantially equal to the thickness of the haze,progressively reducing and then terminating the action of the solutionby the application of additional quantities of the wetting agent to thesurface, together with abrasive powder, and abrasive grinding thesurface to the polish desired while continuing to add the wetting agentand abrasive powder to the surface.

4. That method of removing haze and the like from a surface of atransparent cross-linked methyl methacrylate sheet which includes thesteps of applying to the surface of the sheet a solution containing awetting agent and an acrylic monomer, leaving the solution in contactwith the surface for a period of time sufiicient to swell the sheet adistance approaching the thickness of the haze, and abrasive grindingthe surface to the polish desired in the presence of additional wettingagent and fine abrasive powder.

5. That method of removing haze and the like from a surface of atransparent cross-linked methyl methacrylate sheet which includes thesteps of applying to the surface of the sheet a solution containingkerosene and an acrylic monomer, leaving the solution in contact withthe surface for a period of time suflicient to swell the sheet adistance approaching the thickness of the haze, progressively reducingthe solvent action of the solution by the application of additionalquantities of kerosene to the surface, together with abrasive powder,and abrasive grinding the surface to the polish desired in the presenceof additional kerosene and abrasive powder.

6. That method of removing haze and the like from a surface of atransparent cross-linked methyl methacrylate sheet which includes thesteps of applying to the surface of the sheet a solution containing awetting agent and an acrylic monomer, and abrasive grinding the surfaceto the polish desired while progressively reducing the solvent action ofthe acrylic monomer by the addition of the wetting agent only to thesheet plus the addition of fine abrasive.

References Cited in the file of this patent UNITED STATES PATENTS1,621,780 Hirsh Mar. 22, 1927 2,210,728 Orfald Aug. 6, 1940 2,327,495Budner Aug. 24, 1943 FOREIGN PATENTS 281,751 Great Britain Nov. 28, 1927

6. THAT METHOD OF REMOVING HAZE AND THE LIKE FROM A SURFACE OF ATRANSPARENT CROSS-LINKED METHYL METHACRYLATE SHEET WHICH INCLUDES THESTEPS OF APPLYING TO THE SURFACE OF THE SHEET A SOLUTION CONTAINING AWETTING AGENT AND AN ACRYLIC MONOMER, AND ABRASIVE GRINDING THE SURFACETO THE POLISH DESIRED WHILE PROGRESSIVELY REDUCING THE SOLVENT ACTION OFTHE ACRYLIC MONOMER BY THE ADDITION OF THE WETTING AGENT ONLY TO THESHEET PLUS THE ADDITION OF FINE ABRASIVE.